It is known to store liquids in caverns resulting from the removal of brine from salt deposits. During this storing operation, the cavern is worked as a system which is open to the surface, called "open system" for short, whereby the cavern is not yet provided with a permanent gas- and liquid-tight seal.
There occurs a decrease in the volume of such salt caverns because of the convergence of the surrounding salt deposits. The convergence of a cavern volume (K in %) in a salt deposit as a function of time (t in months) is discussed by Dreyer in "Kali und Steinsalz" 5 (1971), pages 473 to 478 employing a mathematical equation. As an example, the author calculates that for a salt cavern with a storage volume of 15,000 m.sup.3 and having an average depth of 725 m, a cavern height of 100 m, a cavern diameter of 20 m, a cavern temperature of 37.degree. C., an average depth pressure of 170 kp/cm.sup.2 and a liquid internal pressure of 83 kp/cm.sup.2, the volume convergence of K=0.0555.t.0.94 in %. The convergence rate amounted to 0.26 m.sup.3 /day after one month and 0.23 m.sup.3 /day after 7 months.
With a thickness of the salt suspended above the cavern ridge of 150 m, the vertical displacement of the overhang of the salt amounts to 1 mm/month. If the ceiling rock were to consist of rock salt, there would be a vertical displacement on the earth's surface of 0.125 mm after one month.
If one were to consider a sphere as the idealized volume, by introducing a large volume of liquid such as sea water, the hydrostatic external pressure on the surface of the sphere would be increased in proportion to the depth of the sphere.
This pressure is uniformly exerted on the surface. This external pressure can be absorbed by a solid material such as by a steel sphere of corresponding wall thickness. Thus the internal pressure remains constant.
For a cavern volume in a salt deposit, the wall of the spherical cavity is formed of the surrounding salt rock.
This applies also for pear-shaped or cylindrical cavern volumes. However, in such cases the external pressure of the cavern volume is not determined. A convergence, or a volume contraction, occurs in caverns filled with air or water or aqueous salt solutions, in systems open to the surface. This convergence is less for "tougher" plastic rock salt, as calculated by Dreyer.
For storage in salt caverns over a long period of time or for terminal storage, it is necessary to avoid cracking of the ground in order to minimize the convergence until this comes to a standstill by equalization of the pressure.
In this connection, German Published Patent Application No. 2 225 664 discloses a process for deep storage of liquid or oozable radioactive, poisonous wastes, according to which these wastes are mixed above ground before disposal with cement or bitumen to form a slurry and pumped into the cavern. The slurry solidifies in the cavern to form a solid mass which diminishes the convergence. According to this process, small amounts of the wastes are mixed with large amounts of inert fillers with a correspondingly large expenditure for energy and technical apparatus. The cavern fill consists primarily of the filler which itself does not need to be stored.
German Patent No. 25 49 313 describes a process for terminal storage of liquids in salt caverns, whereby the stored liquids are overlaid with a substantially gas- and water-tight covering. Such a medium should consist of a mixture of styrene with cyclohexanol peroxide and cobalt accelerator, which hardens in the cavern. Such an overlying covering, however, can contribute little to lessening the occurrence of convergence.
The concurrently filed U.S. patent application Ser. No. 425,900, now U.S. Pat. No. 4,435,280 issued Mar. 1, 1984, entitled "Process for Storage of Recyclable Liquid Wastes in Underground Salt Caverns" by Lindorfer et al, discloses a process by which these cavities or salt caverns are used for only a limited time as reactors and intermediate storage place, as an open system. Accordingly, the acid components of the liquid wastes are neutralized before, during or after introduction into the cavern and the liquid wastes are allowed to stand for a sufficient length of time to separate the heavier from the lighter specific gravity components, whereby the lighter phase is pumped away for further treatment down to the surface of the heavier phase. The heavy metals, if any, dissolved in the remaining heavier phase are then treated by mixing with alkaline reacting compounds and after removal of the resulting sediment, the remaining salt solution is pumped away down to the surface of the sediment.
By this process, which is carried out in caverns operating as an open system, the question of compensating for convergence does not come up since complete filling of the storage volume can be avoided. Even in the case of complete exhaustion of the storage capacity, convergence is compensated for by pumping out an amount of the stored solution corresponding to the volume decrease and conducting it away or storing it in another cavern.
This process is however not suitable for terminal storage of non-recyclable wastes, since it is not desirable, especially during filling of the cavern, to remove partial amounts of the stored material to compensate for convergence. After complete filling of the storage capacity of the cavern and its final closing, the internal pressure of the stored material is so increased by convergence that it somewhat compensates for the pressure which the salt deposits surrounding the cavern exert on the stored material and thereby a pressure equilibrium is produced. Nonetheless, it can happen that through geological or perhaps technological influences cracks are formed in the surrounding salt deposits whereby the liquid cavern contents can penetrate into these cracks and decrease the solidity of the salt deposits surrounding the caverns in an undesirable way.
If in such caverns for the terminal storage of wastes, such as, for example, contaminated inorganic sludge; crystal-containing salt solutions; lime or gypsum sludge; suspensions of fly ash and flue dusts; sludge from slag production, rubber production, cellulose production; bitumen emulsions, acid resins or tars; sludge from petroleum and the like, are stored, it can lead to unwanted and above all, uncontrollable contamination of the underground.